Safe load signalling devices for construction cranes are known in the art and one example of the patented prior art is U.S. Pat. No. 3,641,551, for SAFE LOAD CONTROL SYSTEM FOR TELESCOPIC BOOM CRANES. Such devices or systems are also known in the art as load moment indicators.
It has also been proposed in the prior art to automatically restrict crane operating control levers against movements in directions which would allow or cause a dangerous load condition in connection with any principal crane function or motion, while allowing freedom of control lever movements in directions which cause reduction of crane loading and therefore do not adversely effect safety. One such prior art patented arrangement is disclosed in U.S. Pat. No. 3,792,780. In this patented system, the load moment indicator, in conjunction with a relay, conditions a solenoid operated valve to control a mechanical actuator which can return manual control levers on the crane to neutral positions. The mechanism automatically restricts the movements of the control levers in directions which would increase unsafe crane loadings while allowing unrestricted lever movements in other directions.
This patented system, while fully operational and reliable, is essentially mechanical in nature and comparatively complex and costly. Also, this mechanical system is not capable of providing control capability for selective operation in each of three different modes, namely, restricting a machine motion control device in one direction, restricting the machine motion control device in the opposite direction, or restricting the machine motion control device in both directions simultaneously.
Therefore, the principal objective of this invention is to improve on the prior art as exemplified in U.S. Pat. No. 3,792,780 by provision of an automatic disabling arrangement for the manual control levers of a crane, the arrangement being considerably simpler and less costly than comparable prior art arrangements, as well as more efficient and involving fewer adjustments. The system embodying the invention is particularly compatible with hydraulic cranes in view of the fact that the system includes hydraulic links and associated pressurized fluid delivery valve means, and therefore is hydraulic in nature, but it is understood that other fluids such as pressurized air may also be used in the system, so it can also be pneumatic in nature.